Generally, an X-ray tube of the rotating anode type comprises a tubular envelope having therein an anode target disc, which is axially rotatable and provided with a radially sloped annular focal track adjacent to its periphery.
The material of the radially sloped annular focal track is generally chosen such as to be comprised of elements having a high atomic number and to have a high melting temperature and low vapor pressure, such as tungsten or a tungsten-rhenium alloy. In other instances, a lower atomic number element or alloy may be used such as, for example, Molybdenum or Titanium-Zirconium-Molybdenum alloy, in order to take advantage of molybdenum characteristic energies in the X-ray beam as they might interact with the object being irradiated. In further instances by way of example, Cerium or Lanthanum borides might be used for similar objectives.
The angle of the radial slope determines the actual irradiated image size and is directly proportional to it. The intensity of the X-rays at the image plane is indirectly but inversely proportional to the angle of the radial slope. In some instances, the radial slope has been arranged such as to have two or more adjacent angles for multiple purpose instruments, by way of example.
A rectangular focal spot area disposed radially on the focal track usually is axially aligned with a linear filamentary cathode. In practice, the cathode may contain two independent linear filaments, generally of differing sizes. The alignment of the filaments in the cathode head is such as to provide electron bombardment from each of the filaments to the same rotating anode focal spot area, a condition called superimposition. The rectangular focal spot area is radially aligned with an X-ray transparent window in the tube envelope. Due to the rotation of the target disc, the surface of the focal track in the focal spot area is constantly changing, thus providing for greater short time interval power than X-ray tubes of the stationary anode type.
The thermionically emitting filamentary cathode, a tungsten coil by way of example, is preferred because of electron emission reproducibility and its ability to withstand ion bombardment emanating from or near the anode.
The cathode is electrically isolated from the anode structure by an insulator usually in the form of a part of the envelope structure. In operation, the cathode thermionically emits electrons, which are electrostatically focused and accelerated onto the focal spot area with sufficient energy to generate X-rays. A useful portion of the X-rays radiating from the focal spot area passes in a divergent beam from the tube through the X-ray transparent window in the tube envelope. However, since the window is radially aligned with the focal spot area, the X-ray beam appears to be emanating from a radial projection of the focal spot area, which is generally referred to as the “effective” focal spot of the tube. In this radial projection, the focal spot along the radial direction is foreshortened such that the foreshortened focal spot acts in the aligned radial direction as an approximate point source of X-ray radiation.
An edge portion of the beam emanating from the “effective” focal spot extends along the sloped surface of the focal spot area and consequently acquires a number of characteristics traceable to what may be termed as the “heel effect”. For example, this edge portion of the X-ray beam, as compared to other portions thereof, appears to be emanating from a focal spot of radically different size, configuration, intensity and, because of strong self absorption of the track material, of different beam energy spectral distribution, thereby degrading uniformity of resolution in a radiograph produced by the X-ray beam, for example. As a consequence of the focal spot foreshortening from a projection on a radially sloped annular focal track, there is a significant variation in both intensity and effective focal spot size.
In some instances, two or more separate and independent focal spot areas, displaced from each other, are provided. For example the two focal spot areas might be displaced 65 millimeters, for example, for purposes of stereo irradiation and subsequent stereo imaging. If this is provided in a single X-ray tube, the single conventional target diameter must be greater than a minimum imposed by the spot displacement. This restriction is often met by using two X-ray tubes. In other instances, a displaced focal spot may be utilized for other purposes, such as reconstruction in X-ray three-dimensional computerized tomography.
The emitted electron beam and/or the emergent X-ray beam can be and are often modulated. The modulation can be in size as in differing focal spot dimensions for imaging gross or fine detail for example, or temporally as sequential bursts of emission synchronized with filming of multiple sequential images, as in angiography and cineradiography, for example, or in energy changes in the X-ray beam energy distribution, as in some bone densitometry. In those instances where multiple beams are used it is often necessary to know which focal spot is doing the irradiation. Herein the modulation can take a number of forms. The foci may be turned active and inactive in a variety of sequences, for example. For foci that are active simultaneously, the emitting intensity of each may be varied at an identifying frequency discernable through a demodulating filter.
The rotating anode tubes generally operate at higher short term intensities by spreading the heat over a greater area than that of the focal spot area and by storing a portion of the heat energy during the short generation time to be dissipated later. The material of the body of the rotating anode disc is chosen such that it provides efficient storage of the thermal energy produced during the short generation time. Generally the heat from the anode target disc is dissipated by means of radiation through the tube envelope and into surrounding electrically and thermally insulating fluid, which transfers the heat energy through the safety housing shield to the ambient surroundings. Care must be taken to shield the radiation and conductive paths from the target to the bearing structure to assure that the maximum bearing temperature is not exceeded.
In some applications, the electrical power supply and control systems for the X-ray tube are amalgamated into one integral package with the X-ray tube, sometimes referred to as a “monoblock system”. This provides for ease of assembly in rapid tomographic systems, by way of example, as well as simplification and weight reduction of the X-ray generating system.